Typically, wind turbines are connected to an electric utility grid in order to be able to generate and supply electric power to consumers located remotely from the wind turbines. The power is sent through the transmission or distribution lines of the utility grid to homes, businesses and so on.
Wind turbines and other electric power generating means connected to a utility grid are protected from malfunctions in the utility grid by grid disconnection switches.
Malfunction in the utility grid may e.g. be                transients or “spikes” which are very short-duration over-voltage and over-current power surges. The most dramatic transients are caused by strokes of lightning, but a majority are caused by large power loads switching on and off.        sags or “brownouts” which are among the most commonly recorded power disturbances, and may occur as a momentary voltage drop.        power interruptions, power outage, or “blackouts” which are the full loss of power in the utility grid.        
The switches disconnect the wind turbines from the utility grid at the detection of the malfunction. The malfunction may be defined as grid variations above some specific limit e.g. voltage drops above +/−5% in relation to the nominal value of the grid voltage.
A problem with the grid disconnection of the wind turbines is the fact that the voltage variation may increase in size or duration by the loss of electric power production from the wind turbine generators. Further, the disconnected wind turbines require a period of time before they can be connected to the utility grid again. The disconnection of the wind turbines affects the production of power from the wind turbines and thus their profitability.
One of the objects of the invention is to establish a method and control system for controlling a wind turbine during malfunction in an electric utility grid without the above-mentioned disadvantage. It is especially an object of the invention to create a method and system which improve the strength and stability of the utility grid during malfunctions as well as the profitability of the connected wind turbines.